This invention relates to a method and apparatus for producing a surface layer on a metallic workpiece.
For the most various reasons, workpieces have been provided wholly or partially with surface layers. For example, surface layers have been provided at places on workpieces which are particularly heavily loaded thermally, chemically or mechanically. These layers may moreover be produced by alteration of the structure of the surface regions of a workpiece, e.g., by hardening by, say, heat treatment, or by the incorporation of foreign substances. Surface layers are also generated by the application of actual surface layers, such as by melting them on or by galvanic surface treatment.
In the case of coated workpieces which are subjected, e.g., to alternating thermal loading, the life of surface layers suitable in themselves is inadequate for certain applications. For example, the surface layer or parts thereof may loosen from the workpiece. In other cases, cracks may form in the surface layer which lead to a breakdown of the surface layer and also of the workpiece.
By way of example, European Patent Application 0 190 378 describes a method for surface-alloying metal with a high-density energy beam and an alloy steel. In one described embodiment, a second heat source comprising a burner or an induction heater is disposed in front of and/or behind the irradiation area of the high-density energy beam. This second heat source provides a supplementary heat source for decreasing the energy of the high-density beam and/or for enhancing the working ability and is intended to prevent thermal cracks. However, such a technique still allows the overall workpiece to be subjected to alternating thermal loading. That is to say, while the area under the influence of the second heat source and high density energy beam, i.e. a laser beam, is significantly heated, the adjacent areas of the workpiece are not. Thus, there is an uneven temperature over the working area of the workpiece. Hence, residual stresses occur during the thermal treatment of workpieces. These residual stresses, in turn, may result in deformations such as localized buckling, within the workpiece and in the surface layer thereon.
Accordingly, it is an object of the invention to improve the quality of a surface layer on a workpiece.
It is another object of the invention to provide a relatively simple technique for forming high quality surface layers on a workpiece.
It is another object of the invention to form relatively thin surface layers on a workpiece with minimal melting of the workpiece.